Recently, attention has been focused on wind power generation systems as a clean energy source that emits no greenhouse gases during power generation. In a wind power generation system, wind turbine blades rotate axially by means of wind force, and the rotational force is converted into electricity to produce electrical output power.
The electrical output power of the wind power generation system is expressed as the product of the shaft-end output power (output power generated by the blades) and the conversion efficiency (efficiency of components, such as a bearing and a generator). The shaft-end output power is expressed by the following equation, indicating that blades having a higher efficiency and a larger diameter generate more electricity:Shaft-end output power=½×air density×wind speed3×blade efficiency×π×(blade diameter/2)2 
The blade efficiency is limited to about 0.5 because of the theoretical upper limit (Betz limit=0.593) and, in practice, the wake effect and the air resistance of the blades. It is therefore difficult to achieve any significant improvement in blade efficiency.
To generate more electricity, it is effective to increase the blade diameter because the output power depends on the square of the blade diameter. A larger blade diameter, however, results in a higher aerodynamic load (the thrust acting in the inflow direction and the moment transferred to the blade roots), which poses the possibility or tendency of increasing the size, the weight, and consequently the cost of equipment, such as a rotor head, a nacelle, and a tower. Accordingly, there is a need for a technique for designing a longer blade without substantially increasing the aerodynamic load on the blade. To circumvent the problem of the increased load, one aerodynamically (geometrically) possible approach is to reduce the projected area of the blades by reducing the chord length (the length of the blade chord) (i.e., by increasing the aspect ratio or decreasing the solidity), thereby reducing the aerodynamic load.
Here, the aspect ratio and the solidity are expressed by the following equations:Aspect ratio=blade length2/projected area of blade   (1)Solidity=total projected area of blades/sweep area of blades=(number of blades×mean chord length)/(π×(blade diameter/2)2)  (2)
In general, a wind turbine blade has a certain optimum chord length for a certain tip speed ratio, as expressed by the following equation (Wind Energy Handbook, John Wiley & Sons, p. 378):Copt/R×λ2×CLdesign×r/R≈16/9×π/n  (3)where Copt is the optimum chord length, R (blade radius) is one half of the blade diameter, λ is the design tip speed ratio, CLdesign is the design lift coefficient, r is the radial position of the blade section, and n is the number of blades.
The design tip speed ratio is the blade tip speed divided by the infinite upstream wind speed. The design lift coefficient is the lift coefficient at the angle of attack at which the airfoil (blade section) has the maximum lift-to-drag ratio (lift/drag) and is determined by the (aerodynamic) profile of the airfoil (blade section) and the inflow conditions (Reynolds number).
FIG. 26 illustrates the definition of the Reynolds number used herein. As shown in the figure, the Reynolds number of a wind turbine, which takes into account the relative wind speed in a certain cross-section A-A of a blade that rotates at a certain rotational speed, is expressed by the following equation:Reynolds number=air density×relative wind speed for blade section×chord length of blade section/viscosity coefficient of air
To maintain the aerodynamic efficiency of a blade, it is desirable that the airfoil (blade section) have the following characteristics:
1. High design lift coefficient
2. Optimum “combination” of design lift coefficients, where the “combination” of design lift coefficients refers to a combination of design lift coefficients of a series of airfoils (airfoil series, family, or set) with different thickness ratios (the percentage obtained by dividing the maximum thickness by the chord length) applied to one wind turbine blade. An example of a combination of thickness ratios of airfoils applied to a wind turbine is 12%, 15%, 18%, 21%, 24%, 30%, 36%, and 42%.
PTL 1 below discloses a series of airfoils for improved wind turbine output power. Specifically, it discloses a series of airfoils having thickness ratios ranging from 14% to 45% and design lift coefficients ranging from 1.10 to 1.25 (see claim 1).
PTL 2 below specifies the profile of the leading edge to reduce a performance drop due to the roughness of the leading edge (e.g., debris and scratches on the leading edge and manufacturing errors). Specifically, it specifies that the percentage obtained by dividing the distance from the chord on the suction side, at a 2% position, by the chord length, where the position of the leading edge along the chord length is defined as 0% and the position of the trailing edge along the chord length is defined as 100%, be 7% to 9%.